The previous Progress Report indicates "of particular interest is the concomitant uptake of methylene blue within the endothelial cells of the vessels" when sensitized lymphocytes were injected directly into the microvasculature. This particular finding has been unique to our laboratory. In discussing this finding with numerous other investigators in the field, the demonstration of endothelial cell staining has become of great interest. To further document that the immune lymphocyte alters the endothelium of the recipient microvasculature, we have undertaken several additional refinements. These studies are currently underway and include the introduction of alloimmune serum with methylene blue, the introduction of the barbitol buffer supernatant plus the methylene blue, and the sensitized lymphocytes with the methylene blue without incubation. Injections of autologous lymphocytes (21 assays), nonsensitized allogeneic lymphocytes (24 assays) and sensitized allogeneic lymphocytes (48 assays) have been convincing. In no instance was endothelial staining by the methylene blue noticed following injection of autologous or non-sensitized allogeneic cells. Injection of the sensitized allogeneic cells with methylene blue has caused distinct staining of the endothelium in 75% of the assays. Furthermore, the injection of alloimmune serum with methylene blue and the injection of barbitol buffer supernatant with methylene blue have not produced any endothelial cell staining. These findings suggest that the sensitized lymphocyte modifies the endothelium of the recipient vessels within 5 minutes of the introduction of the cells. The uptake of methylene blue by the endothelial cell appears to be a specific effect of the lymphocyte rather than immune serum. These unusual findings are compatible with the impressions of many clinicians and investigators in this area and further confirm the role of the immune lymphocyte in the allograft rejection.